Over the last decade, the advent of solid-state lighting has led to rapid advances in the production of high brightness Light Emitting Diodes (LEDs). LEDs hold the promise for a cost-effective solution for ever-increasing illumination-related energy needs. With advanced LED technology, the energy consumption can be reduced significantly.
LEDs brightness is now competing with incandescent and fluorescent light sources: larger chip-size and higher drive-current are keys to the latest improvements. Core-Technological breakthroughs, as well the innovative implementation of optical, electrical and thermal management methods have made these possible. Two aspects of these breakthroughs played a major role:                Higher light extraction chip structures.        Higher thermal-conductivity substrates.        
The light extraction efficiency reflects the ability of photons emitted inside the LED chip to escape into the surrounding medium. For example, the index of refraction of Gallium phosphide-based materials is close to 3.4, compared with 1 for air and 1.5 for epoxy. This results in a critical angle of 17° for air and 25° in epoxy, respectively. If a single interface is considered only 2% of the incident light into air and 4% into epoxy will be extracted. As a comparison, the index of refraction of Gallium nitride-based materials is close to 2.3. This results in a critical angle of 26° into air and 41° into epoxy. If a single interface is considered only 5% of the incident light into air and 12% into epoxy will be extracted. The rest is reflected into the semiconductor where it will eventually be reabsorbed or recycled and results in the performance degradation of the device.
While light extraction efficiency is an important consideration in the design of LEDs, other factors may also be important. For example, to ensure that the entire active layer in the LED is utilized in light emission, it is desirable to spread the electrical current to the entire active layer. To enhance the efficient use of electrical current in light generation, the ohmic contact resistance with the LED should also be as low as possible. To enhance light extraction, the layers between the active layer and the emitting surface of the LED should have high light transmission characteristics. In addition, in order to efficiently reflect light generated by the active layer traveling in directions away from the light emitting surface of the LED, the different layers of the light reflector employed should have high index contrast.
One type of reflectors for LEDs is proposed in the paper “Omni-Directional Reflectors for Light-Emitting Diodes,” by Jong Qyu Kim, et al. Proc. of SPIE Volume 6134, pages D-1 to D-12, 2006. In FIG. 5 of this paper, a GaInN LED with an omni-directional reflector (ODR) is shown. This LED structure comprises a sapphire substrate supporting a GaInN LED. A thin layer of oxidized Ruthenium (Ru) is used as a semi-transparent low-resistance p-type ohmic contact. A quarter-wave thick silicon dioxide low-refractive index layer perforated by an array of silver micro-contacts and a thick silver layer are also employed. In section 3.3.3 on page D-9 of this paper, however, the authors Kim, et al. indicated that the above structure of FIG. 5 is disadvantageous because the above design “needs absorptive semi-transparent current spreading layer, such as RuO2, . . . , which leads to a decrease in reflectivity of the ODR.” Furthermore, the refractive index of silicon dioxide is deemed to be not low enough for high refractive index contrast with high-index semiconductor materials, which limits further improvement of light extraction efficiency in GaN-based LEDs.
As an alternative, the authors proposed an ODR structure illustrated in FIGS. 11 and 12 of the paper. In this alternative ODR structure, the oxidized Ruthenium and silicon dioxide layers in FIG. 5 are replaced by an indium-tin oxide (ITO) nanorod low index layer illustrated in FIG. 12 of the paper. However, as illustrated in FIG. 13 of the paper, the ITO nanorod layer provides mediocre ohmic contact characteristics. Moreover, the ITO material reacts strongly with metal, such as silver. When the ITO nanorod layer proposed by Kim, et al. comes into contact with a silver substrate underneath, interdiffusion occurs at the interface which greatly reduces the reflective properties of the resulting structure. This will also greatly reduce the light extraction efficiency of the LED. It is therefore desirable to provide an improved LED structure in which the above-described difficulties are alleviated.
Thermal management has always been a key aspect of the proper use of LEDs. Poor thermal management leads to performance degradation and reduced lifetime of LEDs. With High Power LEDs, the need is more compelling as more heat is generated.
A substrate of high thermal conductivity becomes a necessity. It allows heat generated at the chip level to be transferred efficiently away from the chip through the substrate.
Given that conventional red (AlGaInP) and blue (InGaN) LED are grown from N+ GaAs and sapphire substrates, respectively, one of the major drawbacks of GaAs and sapphire are their poor thermal conductivity. GaAs and sapphire have thermal conductivity values of 50, and 40 w/m° K roughly, respectively. Obviously, replacing GaAs or sapphire with a carrier of high thermal conductivity such as one made of Si (150 W/m° K) or Cu (400 W/m° K) can significantly improve the LED performance through better heat dissipation.
The substrate from which the LED is grown is referred to herein as the growth substrate. When the growth substrate is replaced by a substitute substrate, such as one made of Si or Cu, the LED epitaxial layers may be subject to change in stress conditions, which may damage the LED. This problem is explained, for example, in U.S. Pat. No. 7,105,857 (“the '857 patent”). As explained in the '857 patent, while the existence of stress per se may not damage the LED, a change in stress conditions may. This is illustrated, for example, in FIGS. 2A-4D of the '857 patent.
Thus, in the case of GaN-based LEDs grown from sapphire substrates, the fabrication process introduces considerable stress on the LED. When the sapphire substrates are replaced, the LEDs may be damaged if such stress is released rapidly. Stress management is thus an important issue. This is due to the fact that the materials in contact with one another in the fabrication process of LEDs have coefficients of thermal expansion (referred to herein as CTE) that can be very different. Hence, when the materials is processed so that they undergo large temperature changes, they will expand and contract by very different amounts. Since these materials are in intimate contact, this uneven expansion and contraction potentially introduce enormous stress in the LED fabricated.
The stress due to CTE mismatch can be calculated with the following equation:σ=E(α1−α2)(T−T0)/(1−ν)where σ is the stress, E is the Young's modulus, α1 and α2 are the CTE of metal such as Nickel and Copper etc. and epitaxial layer GaN, T and T0 is the chip operating and process temperature such as die attach process (for example 250° C.), and ν is Poisson ratio (Nickel and GaN are 0.31 and 0.23, respectively). For example, sapphire has a CTE of 5.0-5.6, and GaN has a CTE of 5.6. While the CTEs of the two materials do not differ by much, during the fabrication process the temperature change is of the order of a thousand degrees, so that the compressive stress generated in the LED can be of the order of about −1.2 GPa.
Solutions have been proposed for stress management. In the '857 patent, for example, the materials with the appropriate CTE are chosen as explained in columns 3-5 of the '857 patent. Such technique, will however, limit the type of materials that can be used, which is not desirable.
It is therefore desirable to develop different stress management systems which does not have the above short comings.